Improve freelist walking

[ksen-lin]

Hi!
I came across the following situations that libslub couldn´t handle:

1. a loop in a freelist led to libslub hanging with ^C being the only help
2. an address in a freelist that couldn't be dereferenced (eg unmapped) resulted in an exception

For the first one i've added a limit that depends on a maximum capacity of a particular slub. For the second i've added a try-except, and also an explicit check to point non-kernel addresses.

With my fixes it looks like this:

1. circular (2 elements) freelist
   
2. invalid next
   

Looking forward to comments/suggestions if you have any. Thanks!